Because a poll asking a direct question is a hell of a lot more accurate in gauging how the population feels about the issue.

Political parties can lose elections for their stances/actions outside their main one -- which seems to have been the case per the actual person from Taiwan that responded to your comment. It doesn't matter what a party is called or what their main goals are if they're bad at their job.

If and when the people of Taiwan decide they want reunification, it will happen. Thankfully Beijing isn't going to be allowed to force the issue.